Seismic sensor devices, systems, and methods including noise filtering

ABSTRACT

Methods are disclosed for sensing acoustic waves in a medium. One example includes a first elongated member, a first motion sensor sensitive to vibrations of the first elongated member, a second motion sensor spaced apart from the first motion sensor and also sensitive to vibrations of the first elongated member, and a first vibration source operably coupled to the first elongated member and configured to vibrate the first elongated member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/774,878, filed on Sep. 11, 2015, and issued Dec. 12, 2017 as U.S.Pat. No. 9,841,519, which is a national stage application of PCTApplication No. PCT/US2014/028162, filed on Mar. 14, 2014, entitled“Seismic Sensor Devices, Systems, and Methods Including NoiseFiltering,” and claims the benefit of and priority to U.S. ProvisionalApplication No. 61/785,354, filed on Mar. 14, 2013, entitled “SeismicSensor System With Streamer Noise Rejection,” the entire contents ofeach of which are hereby incorporated by reference herein for allpurposes.

BACKGROUND

In towed marine seismic exploration, a hydrophone array is towed behinda marine vessel 20 near the sea surface 22, as in FIG. 1. Thehydrophones are mounted in multiple sensor cables commonly referred toas streamers 24. The streamers serve as platforms for the hydrophones. Aseismic sound source 26, also towed near the sea surface, periodicallyemits acoustic energy. This acoustic energy travels downward through thesea, reflects off underlying structures or subsea strata 28, and returnsupward through the sea to the hydrophone array. Reflected seismic energyarrives at towed-array receive points. The hydrophone array containsmany such receive points and records the upward traveling seismicacoustic wavelet from the seabed 30 at each of the receive points. Thehydrophone recordings are later processed into seismic images of theunderlying structures.

Noise is a major consideration in towed streamer operations. Noisesources include swell noise and wave noise from the sea surface. Andtowing the streamer through the water causes noise. Some of this noisepropagates through the streamer and some through the water columnitself. The typical way of dealing with noise sources is to use acombination of temporal and spatial filtering. Temporal filtering isaccomplished by discrete digital sampling of the hydrophone signals intime with weighting applied to the samples. The hydrophone channels alsoinclude analog filters to prevent aliasing of signals at frequenciesgreater than half the sample rate. The spatial samples are typicallyformed by group-summing individual hydrophone outputs so that pressurenoise propagating along the length of the streamer is attenuated. Thisspatial sampling has no impact on noise that propagates in a directionorthogonal to the streamer axis. Typical hydrophone groups consist ofeight or so hydrophones in a 12 m section of the streamer.

Acoustic impedance, ρc, is the product of the density and the speed ofsound in a medium. Reflection of at least some of the sound-wave energyoccurs whenever a change in acoustic impedance is encountered by thesound waves. The energy that is not reflected is transmitted (refracted)beyond the boundary between the two regions of different acousticimpedances. The pressure waves are compression waves, which induceparticle motion in the direction of propagation. At a planar interfacebetween two different homogenous media, a sound wave reflects at anangle equal to the angle of incidence θ₁ and refracts at an angle θ₂.The refraction angle is given by:θ₂=sin⁻¹(c ₂ sin θ₁ /c ₁).

The subscript refers to the sound wave moving from medium 1 to medium 2and c₁ and c₂ are the speeds of sound in each medium. If the incidentangle this zero, then the refracted energy propagation path will be atan angle θ₂ of zero.

For an incident angle θ₁ of zero and no energy converted to shearenergy, the reflection coefficient at the water-air interface isdescribed by:R _(pp)=(ρ₂ ·c ₂−ρ₁ ·c ₁)/(ρ₂ ·c ₂+ρ₁ ·c ₁)≈−1.

The reflected energy at the water-air interface is R² _(pp), or nearly1, making the sea surface a near perfect reflector of sound energy.After returning from the sea bottom or the target of interest, theenergy is again reflected by the sea surface back to the streamer.Because a typical hydrophone has an omni-directional response, thehydrophone array also records a ghost response, which is the seismicacoustic wavelet reflected from the sea surface and arriving delayed intime and reversed in polarity. The ghost is a downward-traveling seismicacoustic wave that, when added to the desired wave, detracts from therecorded seismic image. The ghost-causing reflection can also continueto the sea bottom or other strong reflector and be reflected back up toagain interfere with the desired reflections and further degrade theimage. These reflections are commonly referred to as multiples.

For a vertically traveling pressure wave, the ghost produces a notch inthe frequency spectrum of a hydrophone response at f_(notch)=c/2d, wherec is the speed of sound and d is the streamer depth. Seismic streamershave been conventionally towed at a depth of 10 m or less. At a depth of10 m, the notch frequency (f_(notch)) is 75 Hz. A frequency responseextending beyond 100 Hz is required for high seismic image resolution.Because the notch frequency is inversely proportional to the tow depth,streamers are often towed at shallower depths to improve the resolutionof a seismic image. Towing at shallow depths is problematic becausenoise from the sea surface begins to interfere with the desired seismicsignals. These effects are worsened as weather deteriorates, sometimescausing the crew to discontinue operations until the weather improves.The elimination of ghost-notch effects would enable towing at greaterdepths farther away from surface disturbances.

Ocean bottom systems, in which the seismic sensors are placed on theseabed, reject ghosts and multiples by a technique commonly known as p-zsummation. In an acoustic wave, the pressure p is a scalar, and theparticle velocity u is a vector. A hydrophone, with a positiveomni-directional response, records the seismic acoustic wave pressure p.A vertically oriented geophone or accelerometer records the verticalcomponent of the seismic acoustic-wave particle velocity u_(z), with apositive response to up-going signals and a negative response todown-going signals. In p-z summation, the velocity signal is scaled bythe acoustic impedance ρc of seawater before it is added to the pressuresignal. A gimbaled single-axis sensor is also scaled to account for thechange in sensitivity of the particle-motion sensor due to the off-axisarrival of any received signals. If an accelerometer is used, its outputsignal can be integrated to obtain the velocity signal, or thehydrophone signal can be differentiated so that it can better spectrallymatch the accelerometer. This produces a compound sensor that has a fullresponse to the upward traveling wave and at least a partially mutedresponse to the downward traveling wave to reject the ghost andmultiples. One such method of signal conditioning and combination ofsignals to get a single de-ghosted trace is described in U.S. Pat. No.6,539,308 to Monk et al. FIG. 2 is a two-dimensional (2D) representationof the response of a particle-velocity sensor. FIG. 3 is a 2Drepresentation of the response of an omni-directional hydrophone summedwith the response of a vertical particle-motion sensor. The fullthree-dimensional responses can be envisioned by rotating the 2Dresponses about their vertical axes.

Operating a particle-motion sensor in a seismic streamer presents aproblem because the streamer experiences accelerations due to towing orsea surface effects that are large compared to accelerations caused bythe desired seismic reflections. Moreover, these unwanted accelerationsare in the same spectral band as the desired reflection response. When atowing vessel encounters ocean waves, there are small perturbations inthe speed of the vessel. The vessel also typically undergoes a yawingmotion. FIG. 4 depicts energy being imparted to the streamers 24 byspeed variations 32 and yawing motion 34. FIG. 5 is a side viewdepicting energy causing accelerations and transverse waves in thestreamer 24. (The energy's effect on the streamer is exaggerated in FIG.5 for illustrative purposes.) Most of the energy is attenuated byelastic stretch members 36, typically in front of the sensing arrays.While the energy is greatly attenuated, some does remain. Accelerationsa caused by planar pressure waves due to the desired seismic reflectionsare given by:a=p·2·π·f/Zwhere p is the acoustic sound pressure level, f is the frequency, and Zis the acoustic impedance.

Performance of a particle-velocity measuring system may be near theambient noise limits. Typically, seismic-data customers require ambientnoise from streamer hydrophone systems to be below 3 μbar. Since theacoustic impedance of seawater is 1.5 MPa·s/m, a 3 μbar pressure wave at4 Hz produces particle accelerations of roughly 0.5 μg. FIG. 6 shows amechanical model of the frequency response of typical cable axialaccelerations in the middle of a streamer. The presence of a secondarypeak at 4 Hz, only 1.5 orders of magnitude below the primary peak,indicates that, in some cases, the cable dynamic motion can be greaterthan the seismic signal to be measured.

U.S. Pat. No. 7,167,413 to Rouquette uses an accelerometer in a seismicstreamer to reject the ghost-notch effect. Rouquette uses a mass-springsystem to reduce the effect of cable dynamics on the accelerometer and aload-cell system to measure and reject the cable-motion-induced noise onthe accelerometer. The Rouquette system relies on well-known complexmechanical relationships that do not remain constant with manufacturingtolerances, aging, and environmental conditions. Rouquette uses asignal-processing adaptive algorithm to derive the relationship of theload-cell-sensor-and-mass-spring system to the acceleration acting onthe accelerometer in situ. Rouquette describes a complex mechanical andelectronic system.

U.S. Pat. No. 7,239,577 to Tenghamn et al. describes an apparatus andmethod for rejecting the ghost notch using an acoustic-waveparticle-velocity sensor. Tenghamn et al. relates to the use of afluid-damped, gimbaled geophone. The fluid encapsulating the geophone ischosen to provide damping of the sensor swinging on its gimbals. Whilenot described in Tenghamn et al., it is known in the art that amass-spring vibration-isolation system can reduce the effect of cablemechanical motion on the geophone response. Motion of the geophonecaused by cable mechanical motion may be indistinguishable fromacoustic-wave particle motion in the geophone response. The seismic-waveparticle motion of interest may be obscured by cable mechanical motionin Tenghamn et al. This technique also gives the response similar to thecardioid in FIG. 3, where there are still undesired signals coming fromthe surface and being induced by streamer excitation along the streameraxis.

U.S. Pat. No. 7,359,283 to Vaage et al. involves a method of combiningpressure sensors and particle-motion sensors to address the impact ofmechanical motion on the particle-motion sensors. In this method, theresponse of the particle-motion sensor below a certain frequency f₀ isnot used but only estimated from the pressure-sensor response and knownpressure-sensor depth. The frequencies rejected are those for whichmechanical motion of the streamer is expected. The estimated responsehas poor signal-to-noise ratio at the lower frequencies of interest.This rejection below a certain frequency is not optimal as it alsorejects valid signals in an important low-frequency band wheredeep-target data is likely to exist.

While these patents all describe methods to reject the ghost notch in aseismic streamer, none adequately addresses the effects of streamer towand other noise that affects the particle-motion sensor or hydrophonemeasurements. All also fall short of producing high-fidelity, sensedacoustic-wave components with good signal-to-noise ratio down to thelowest frequencies of interest.

SUMMARY

Implementations of the present invention provide systems, methods, andapparatus for sensing seismic signals in a marine environment. Oneexample system for sensing acoustic waves in a medium includes a firstelongated member, a first motion sensor sensitive to vibrations of thefirst elongated member, a second motion sensor spaced apart from thefirst motion sensor and also sensitive to vibrations of the firstelongated member, and a first vibration source operably coupled to thefirst elongated member and configured to vibrate the first elongatedmember.

In some embodiments, the system further includes a processing deviceoperably coupled to the first motion sensor and the second motionsensor, the processing device configured to calculate a transferfunction at least partially based on information received from the firstmotion sensor and the second motion sensor while the vibration sourcevibrates the first elongated member, the processing device being furtherconfigured to calculate a filtered acoustic wave signal based at leastin part on the calculated transfer function. The system may also includea third motion sensor, where the processing device is further configuredto calculate the transfer function at least partially based oninformation received from the third motion sensor while the firstvibration source vibrates the first elongated member. The third motionsensor may also be sensitive to vibrations of the first elongatedmember. The first motion sensor and the third motion sensor may belongitudinally spaced apart along the first elongated member, and thesecond motion sensor may be positioned between the first motion sensorand the third motion sensor. The system may further include a secondvibration source operably coupled to the first elongated member andconfigured to vibrate the first elongated member.

In some embodiments, the first elongated member may be a stress member.The first vibration source may be coupled to the first elongated memberin a manner that vibration of the vibration source producescorresponding vibration of the first elongated member. The firstvibration source may include a motor with an eccentrically loaded shaft,and a rotation axis of the motor may be oriented approximatelylongitudinally relative to the first elongated member. The vibrationsource may be configured to vibrate the first elongated member in atransverse direction relative to the first elongated member.

An example method of sensing acoustic waves in a medium may include theacts of receiving readings from a first motion sensor and from a secondmotion sensor, the first motion sensor and the second motion sensor bothcoupled to a streamer, receiving readings from a third motion sensoralso coupled to the streamer, the third motion sensor positioned betweenthe first motion sensor and the second motion sensor, and filteringnoise from the readings received from the third motion sensor togenerate a set of filtered data. The filtering noise may include theacts of determining first and second transfer functions corresponding tothe streamer, modifying the readings received from the first motionsensors using the first transfer function to generate a first set ofmodified readings, modifying the readings received from the secondmotion sensor using the second transfer function to generated a secondset of modified readings, and modifying the readings received from thethird motion sensor using the first and second sets of modified readingsto generate the set of filtered data.

In some embodiments, the first and second transfer functions may bedetermined prior to receiving the readings from the first, second, andthird motion sensors. The determining of the first and second transferfunctions may include the acts of introducing first vibrations into thestreamer using a first vibration source, sensing the first vibrationsusing at least the first and third motion sensors, introducing secondvibrations into the streamer using a second vibration source, sensingthe second vibrations using at least the second and third motionsensors, and calculating the first and second transfer functions basedon the sensing of the first and second vibrations.

In some embodiments, modifying the readings received from the firstmotion sensor using the first transfer function may include multiplyingthe readings received from the first motion sensor by the first transferfunction, and modifying the readings received from the second motionsensor using the second transfer function may include multiplying thereadings received from the second motion sensor by the second transferfunction. Further, modifying the readings received from the third motionsensor may include subtracting the first and second sets of modifiedreadings from the readings from the third motion sensor. In someembodiments, the method may further include the act of advancing thestreamer in the medium. The method may include the acts of advancing thestreamer in a first direction, and determining the first and secondtransfer functions corresponding to the streamer advancing in the firstdirection, and also advancing the streamer in a second direction, anddetermining third and fourth transfer functions corresponding to thestreamer advancing in the second direction.

An example method of characterizing a marine seismic streamer mayinclude the acts of delivering first vibrations to a first portion ofthe streamer, the first vibrations propagating from a first locationtoward a first motion sensor and further toward a second motion sensor,receiving first readings from the first motion sensor and from thesecond motion sensor corresponding to the first vibrations, deliveringsecond vibrations to a second portion of the streamer, the secondvibrations propagating from a second location toward a third motionsensor and further toward the second motion sensor, receiving secondreadings from the second motion sensor and from the third motion sensorcorresponding to the second vibrations, and calculating one or moretransfer functions representative of the marine seismic streamer basedon the received first and second readings.

In some embodiments, the method may further include the act of operatingone or more artificial vibration sources to produce the first vibrationsor second vibrations. The one or more transfer functions may berepresentative of physical characteristics of the streamer thatdetermine how noise is transmitted along the streamer.

An example method of sensing acoustic waves in a medium may include theacts of transmitting first vibrations using a first vibration sourcepositioned at a first location along a streamer, sensing the firstvibrations using a first motion sensor and a second motion sensor,transmitting second vibrations using a second vibration sourcepositioned at a second location along the streamer, sensing the secondvibrations using a third motion sensor and the second motion senor,characterizing the streamer based at least in part on the sensing of thefirst and second vibrations, and filtering acoustic wave measurementsfrom the second motion sensor based at least in part on thecharacterization of the streamer. In some embodiments, the first andsecond vibrations may each include a sweep of a plurality of vibrationfrequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. For better understanding, the likeelements have been designated by like reference numbers throughout thevarious accompanying figures. Understanding that these drawings depictonly typical embodiments of the invention and are not therefore to beconsidered to be limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

FIG. 1 is a side elevation view of a typical seismic survey operationshowing an array of hydrophones under tow behind a survey vessel anddepicting rejected seismic energy arriving at towed-array receivepoints;

FIG. 2 is a two-dimensional graph of the response of a particle-velocitysensor;

FIG. 3 is a two-dimensional graph of the response of an omni-directionalhydrophone summed With the response of a vertical particle-velocitysensor;

FIG. 4 is a top plan view of a typical survey as in FIG. 1 depictingtow-speed fluctuations and yaw;

FIG. 5 is a side elevation view of a survey as in FIG. 4 depicting theexaggerated effects of tow-speed fluctuations and yaw on streamer shape;

FIG. 6 is a plot of typical accelerations of a streamer in a survey asin FIG. 1;

FIG. 7 is a schematic diagram of a seismic system including motionsensors according to an embodiment;

FIG. 8 is a frequency-domain schematic diagram of the responses of themotion sensors as in FIG. 7 to the acoustic wave component of incidentacoustic energy according to an embodiment;

FIG. 9 is a frequency-domain schematic diagram of the responses ofmotion sensors of FIG. 7 according to an embodiment;

FIG. 10 is a time-domain plot of the output of a motion sensor of FIG. 7that is responsive to platform motion and acoustic pressure wavesaccording to an embodiment;

FIG. 11 is a time-domain plot of the output of a motion sensor of FIG. 7that is responsive only to platform motion according to an embodiment;

FIG. 12 is a plot of the difference between the plots of FIGS. 10 and11;

FIG. 13 is a schematic diagram of a seismic system according to anembodiment;

FIGS. 14A and 14B are cross-sectional views of a seismic systemaccording to another embodiment;

FIG. 15 is a schematic diagram of a seismic system according to yetanother embodiment;

FIG. 16 is a schematic diagram of a seismic system according to stillanother embodiment;

FIG. 17 is a side view of a seismic system according to an embodiment;

FIG. 18 is a side view of a seismic system according to an embodiment;

FIG. 19A is a schematic illustration of a portion of a streameraccording to at least one embodiment;

FIG. 19B is a cross-sectional view of a streamer including a motionsensor connected to a stress member of the streamer according to anembodiment;

FIG. 19C is a cross-sectional view of a streamer including anothermotion sensor isolated from a stress member of the streamer according toan embodiment;

FIG. 20 is a diagram of an equivalent circuit modeling behavior ofelements and/or components of a streamer according to an embodiment;

FIG. 21 is an exploded view of a vibration source according to anembodiment;

FIG. 22 is a schematic illustration of a streamer, signal processor, anda controller according to an embodiment;

FIG. 23 is a chart of acts for filtering readings of a streameraccording to an embodiment;

FIG. 24 is a chart of acts for calibrating a streamer according to anembodiment;

FIG. 25 is a chart of acts for operating a streamer according to anembodiment;

FIG. 26 is a schematic illustration of an exploration vessel's movementaccording to an embodiment; and

FIG. 27 is a chart of acts for calibrating the streamer according to anembodiment.

DETAILED DESCRIPTION

FIG. 7 is a block diagram of a general version of an underwater seismicsystem 19 according to an embodiment of the disclosure. The underwaterseismic system 19 may involve techniques for using motion sensors orsensor assemblies with different responses to sound-wave-induced signalsand similar responses to platform (e.g., streamer, cable, or autonomousnode), motion to improve the signal-to-noise ratio of data acquired forseismic imaging. In FIG. 7, two motion sensors 40, 41 and one pressuresensor 42, such as a hydrophone, provide signals that may be combined toproduce a noise-reduced and de-ghosted signal (i.e., at least apartially filtered signal or reading). A group of pressure sensors canbe used in lieu of a single sensor, e.g., to reduce the noise arisingfrom pressure waves, which may propagate in a longitudinal directionalong the streamer. The motion sensors may be dc-sensitive and/orcapable of resolving or identifying the gravity vector (i.e., directionand magnitude); otherwise, one or more additional orientation sensor maybe used to determine the orientation of the streamer.

The first motion sensor 40 may have a response to acoustic waves that isideally, but not necessarily, equal to that of the medium in which thestreamer is submerged, such as seawater. Moreover, the response may beincreased beyond that of seawater if more gain is desired. The secondmotion sensor 41 has a response to acoustic waves that is measurablydifferent from that of the first motion sensor 40. This difference inacoustic response may be obtained by using different materials ormaterial composition or the geometric configuration of the sensors. Inone or more embodiments, the material and geometric properties of one ormore sensors may be chosen to match mechanical responses to platformmotion. For example, if one, some, or all of the motion sensors aredesigned to interact with a cable in the same way as a second-ordermass-spring system, then the masses (including added mass, ifappropriate) of the sensors and their associated spring constants aremade equal.

In some embodiments, the readings or signals from the first and secondsensors 44, 45 may combined or processed to produce an improved signal.For example, the first and second outputs 44, 45 of the first and secondmotion sensors 40, 41 may be subtracted at block 46, either locally orafter remote processing, to produce a reduced-noise response signal 48indicating particle motion due to acoustic waves with platform motionattenuated. If the signal of one of the sensors is reversed in phase,combining the first sensor signal and the second sensor signal mayinvolve adding signals instead of subtracting.

Moreover, in some embodiments, the reduced-noise or filtered responsemay be scaled at block 50 to match the pressure-sensor response 52(e.g., the hydrophone signal) and may be used in p-z summation at block54 to produce a final output signal 56 that also may account for andreject ghost notches and multiples. The combining the first sensorsignal and the second sensor signal and the p-z summation may beperformed locally by a general or special purpose computer, includingbut not limited to analog circuitry, digital logic circuitry,algorithmically in a microprocessor, remotely on a shipboard computer,in off-line data processing, etc.

FIG. 8 is a block diagram of the motion sensors 40, 41 of FIG. 7 shownin the frequency domain and indicating their transfer functions to theacoustic wave component 58 of incident energy. The acoustic wavecomponent includes the seismic signals of interest. The first sensor 40and the second sensor 41 may have unequal or different acoustic wavetransfer functions H₁(s) and H₂(s). The transfer function H₁(s) issensitive to acoustic wave particle motion, so that the first sensor 40produces an output response O₁(s) that represents or relates to particlemotion. The transfer function H₂(s) is insensitive or less sensitive toacoustic wave particle motion, and the second sensor 41 has an outputresponse O₂(s) that at least substantially does not include the motionof surrounding acoustic-medium particles.

FIG. 9 is a block diagram of the motion sensors 40, 41 of FIG. 7 in thefrequency domain indicating their transfer functions to theplatform-motion component 59 of incident energy. The transfer functionsH₃(s) and H₄(s) of the two motion sensors 40, 41 to platform motion areproportional (or equal) in magnitude, but could be opposite in phase.Thus, both sensors 40, 41 have similar output responses O₃(s) and O₄(s)to platform motion. The composite transfer functions of the first andsecond motion sensors 40, 41 to incident energy are the combinations ofH₁(s) and H₃(s) for the first sensor and of H₂(s) and H₄(s) for thesecond sensor. The composite responses of the two sensors are thecombinations of O₁(s) and O₃(s) for the first motion sensor and of O₂(s)and O₄(s) for the second motion sensor.

FIG. 10 is an example representation of the time-domain response of thefirst sensor 40 to incident energy that includes both platform motionand acoustic waves. The first sensor's response 44 is sensitive to bothplatform noise and the acoustic wave. FIG. 11 is the correspondingresponse of the second sensor 41 to the same incident energy. The secondsensor's response 45 is sensitive only to the platform-noise componentof the incident energy or at least less sensitive to acoustic waves.FIG. 12 is a plot of the result of combining the responses of the twosensors by subtracting the output 45 of the second sensor from theoutput 44 of the first sensor to produce the noise-subtracted acousticwave signal 48 of FIG. 7. Although, for purposes of simplifying thedescription, the response of the second sensor to pressure waves wastreated as zero, it may have some slight response, or even a negativeresponse, to pressure waves. Furthermore, the first and second sensoroutputs may not be exactly matched to streamer vibrations. In any event,in one or more embodiments, the signal subtraction still results in anacoustic wave response with a greatly attenuated platform-motionresponse that can be scaled and combined with the hydrophone data by p-zsummation.

Various specific versions of the general system indicated in the blockdiagrams of FIGS. 7-9 use different levels of acoustic impedance toachieve the desired difference in response to acoustic wavelets. Asdescribed above, the motion sensors 40, 41 and the pressure sensor 42are mounted in, on, or to a platform (e.g., to a stress member of thestreamer, as described below in further detail). For example, the motionsensors 40, 41 and/or pressure sensors 42 may be enclosed in anunderwater streamer or mounted inside a cable-positioning bird attachedto a streamer. The motion sensors may be isolated acoustically from eachother, but may be located close together and separated into individualregions by a divider, for instance.

The first motion sensor may be enclosed in a first region with anexterior whose acoustic impedance is similar to that of the surroundingseawater so that acoustic waves penetrate the exterior with minimalreflections and act on the sensor. The second motion sensor may belocated in an acoustically opaque enclosure in a second region and maybe at least substantially unaffected by incident acoustic waves. Thestreamer may be under tension (e.g., during operation thereof) and mayhave a small and/or erratic or irregular response to the acoustic waves.Any response of the streamer to the acoustic waves may be recorded asplatform motion. Therefore, in some instances, the first sensor may havea proportional response to acoustic waves, and the second sensor mayhave a negligible response to acoustic waves. Additionally, in someembodiments, the sensors may be calibrated to have matched responses toplatform motions, (e.g., to streamer vibrations), for instance, byequating their masses (including added mass, if appropriate) andassociated spring constants if they behave as second-order mass-springsystems. Subtraction, either locally or after remote processing, of thesecond sensor signal from the first sensor signal accordingly yields thedesired acoustic wave signal with greatly attenuated streamer-motionresponse.

An example of the seismic system of FIGS. 7-9 is shown in FIG. 13 withtwo motion sensors 60, 61, which may be separated acoustically by acentral divider 64. The seismic system may also include a pressuresensor 62. In some embodiments, the first motion sensor 60 may becontained in a first region 66 of the streamer with a rigid,acoustically transparent exterior 68. For example, the exterior 68 maybe a perforated, rigid housing covered with a flexible and/oracoustically transparent skin 70. The interior of the first region 66may be filled with fluid.

In some embodiments, the skin and fluid both may have acousticimpedances equal to that of the surrounding medium, such as seawater. Afirst test mass 72 may have an acoustic response similar to that of thefluid or other medium surrounding the first test mass 72 in someembodiments; in other embodiments, however, the response of the firstmass 72 may be increased beyond that of the surrounding medium (e.g., ifmore gain is desired).

In some embodiments, the first test mass 72 is connected to the exteriorof the streamer by means of a displacement, velocity, or accelerationsensor, which serves as the motion sensor. The first sensor 60 may usethe exterior of the streamer as a frame of reference and may act as aspring in coupling the test mass and streamer dynamically. For example,the first sensor may be single crystal or a PZT bender. If the sensor isa single-axis sensor, multiple test-mass systems may be used to form amulti-axis sensor (e.g., two-axis or three axis sensor), which mayinclude some or all test masses calibrated to match in both acoustic anddynamic response. Alternatively or additionally, an embodiment mayinclude several sensors connected to a common test mass for multi-axismeasurement.

In some instances, the second sensor 61 and a second test mass 73 may beconnected in an assembly in a second region 67 on the opposite side ofthe divider from the first region 66. The second sensor may differ fromthe first sensor in that housing exterior 69 of the second sensor mayhave an acoustic impedance much greater than that of the surroundingmedium. Furthermore, in some examples, the interior 67 of the secondsensor housing may be filled with air to account for any non-negligibleelasticity in the housing exterior 69. Augmenting the effects of theincreased acoustic impedance of the second sensor's housing is itsrigidity, which may allow the housing to act as an acoustic shield,analogous to a Faraday cage in electromagnetism. The acoustic impedanceof the housing exterior 69 may include a material having a suitably highdensity or sound speed.

As shown in FIGS. 14A and 14B, in additional or alternative embodiments,a seismic system may include two sets 80, 81 of motion sensors and apressure sensor 82. For example, the first sensor set 80 and the secondsensor set 81 may be connected to a single rigid body 84 that carriesstreamer vibrations. The rigid body 84 has a large-diameter firstportion 86, a smaller-diameter second portion 87, and a transitionsection 88 joining the first and second portions 86, 87. In at least oneembodiment, the smaller-diameter portion 87 is tubular in shape with aninner side 83 and an outer side 85. The first sensor set 80 may encircleor surround a section of the second portion 87 of the rigid body 84 andmay be connected to outer side 85 of the rigid body 84.

Three or more individual sensors may be used to constitute the first set80. If axisymmetry is not employed, then the first sensor set 80 may belocated alongside the rigid body 84. An acoustically transparentexterior 90, which may consist of a flexible membrane over a perforated,rigid housing, separates the sensor system from the surrounding medium,such as seawater. A first cavity 92, may be located between the secondportion 87 of the rigid body 84 and the exterior 90 and may be filledwith fluid. In some embodiments, the exterior 90 and the fluid haveacoustic impedances equal to the acoustic impedance of the surroundingmedium.

A first test mass 94, with acoustic properties similar to or the same asthe acoustic properties of the first test mass of FIG. 13, may besuspended in the first cavity 92 and may encircle the second portion 87of the rigid body 84. The first test mass 94 may be mechanically coupledto the outer side 85 of the rigid body 84 by the first set 80 of motionsensors with properties that may be similar to or the same as theproperties of the first sensor 60 of FIG. 13, but with the rigid body 84as their frame of reference. In some instances, a second cavity 93 maybe contained entirely within the tubular second portion 87 of the rigidbody 84.

In one or more embodiments, the second cavity 93 contains a second testmass 95 suspended in fluid and coupled to the rigid body 84 by thesecond set 81 of motion sensors connected to the inner side 83 of therigid body 84. The dynamic response of the second set 81 of sensors maybe calibrated to have a response to streamer vibrations that matches theresponse of the first set 80. Unlike the first test mass 94, however, norequirements are placed on the acoustic response of the second test mass95. The rigid body 84 may act as an acoustic shield to the second sensorset 81 and may include a material with relatively high acousticimpedance. A benefit of this coaxial arrangement is that multipleindividual sensors respond to the accelerations of each test mass.Combining the output signals of the motion sensors may produce a moreaccurate reading or estimate of the actual acceleration values. In someinstances, the first and second sensor sets 80, 81 are sensitive toradial motion; an additional test-mass-sensor system may be included ineach cavity in alignment with the streamer axis if tri-axis sensitivityis needed.

As shown in FIG. 15, embodiments may also include a streamer with arigid, acoustically transparent exterior 98, which may have two motionsensors 100, 101, such as dc-sensitive, tri-axis accelerometers, and onepressure sensor 102, such as a hydrophone. The exterior 98 may comprise,for instance, a perforated, rigid housing covered with a flexible,acoustically transparent skin. The accelerometers may includemicroelectromechanical system (MEMS), PZT, single crystal, othersuitable devices or systems, or any combination thereof.

The motion sensors 100, 101 are rigidly mounted to first and secondrigid housings 104, 105 and may directly sense or measure dynamicstreamer motion. Moreover, the motion sensors 100, 101 may beacoustically coupled to the cable exterior 98, but may be acousticallyisolated from each other, for instance, by a central divider 106. Eachof the first and second housings 104, 105 may be constructed and/orconfigured such that the mass of the first housing 104 plus the massenclosed therein is equivalent to the mass of the second housing 105plus the mass enclosed therein.

The dynamic couplings 106 between the housings and the streamer exterior98 may be designed or configured to act as second-order mass-springsystems with equal spring constants so that the equality of themass-spring relationships is preserved. Additionally or alternatively,in some examples, the first and second 104 housings 104, 105 may havedifferent acoustic cross-sections, so as to generate different responsesto acoustic pressure waves. Specifically, in some embodiments, the firstsensor 100 generates a first sensor signal 108 that is a goodrepresentation of the acoustic particle motion, and the second sensor101 produces a second sensor signal 109 that is largely insensitive toacoustic waves. The first and/or second housings 104, 105 may havedifferent geometries, different materials, different cross-sections, orotherwise different configurations, so as to have different transferfunctions for each sensor.

The second sensor signal 109 may be subtracted at block 107 from thefirst sensor signal 108, which may provide a suitable or desiredpressure wave signal with greatly attenuated response to streamermotion. Open-cell foam can be used, for example, to serve as the dynamiccouplings 106 between the first and/or second housings 104, 105 and theexterior 98. Filled with a fluid calibrated to match the acousticimpedance of the surrounding medium, such as seawater, the foam canserve also as a transparent acoustic coupling. In at least one example,the first housing 104 may be sealed with respect to the fluid and filledwith air to account for any non-negligible elasticity in the housing. Insome embodiments, the second housing 105 is perforated or slotted andallowed to fill with the surrounding fluid. Hence, in some instances,the resultant disparity in overall density between the housings mayaccommodate or account for different responses thereof to incidentpressure waves.

A modified version of the seismic system of FIG. 15 may enhance theoverall gain of the system as shown in FIG. 16. The first sensor 110 maybehave acoustically and dynamically similar to or the same as the firstsensor 100 of FIG. 15. In some embodiments, the second sensor 111produces a response to pressure waves that matches the response of thefirst sensor 110 and a streamer-motion response equal in magnitude butopposite in polarity to that of the first sensor 110. The first housing114 and the second housing 115 may be constructed similar to the firstand second housings 104, 105 in FIG. 15, particularly in terms ofacoustic cross-section and density, so that the first and secondhousings 114, 115 have a similar mass-spring response to cable motion,but a measurably different response to incident acoustic pressure waves.

The second housing 115 additionally includes a test mass 116 that may beconfigured or designed to oscillate in a fluid and have an acoustic waveresponse matching the response of the first housing 114. Alternativelyor additionally, the response of the test mass to streamer motion may besubstantially less than the response of the first and second housings114, 115, because the test mass may be suspended in a fluid and thefirst and second housings 114, 115 may be mechanically coupled to thecable exterior. The test mass 116 may be non-rigidly or flexiblyconnected to the second housing 115 via a displacement, motion,acceleration sensor 111, or a combination thereof, which may use thesecond housing 115 as a frame of reference.

In one example, a cantilevered accelerometer, which may includepiezoelectric materials, may be used as the motion sensor. Multipleaccelerometers may be employed to form a tri-axis sensor, with each testmass calibrated to match the acoustic response of the first housing 114along respective axes thereof. Pressure waves, which impart motion onthe test mass 116 but, in some embodiments, not on the second housing115, may be positively detected (i.e., in phase). Accordingly, in atleast one example, pressure signals from the first sensor 110 and thesecond sensor 111 match in both magnitude and sign. Conversely, streamervibrations, which influence the second housing 115 but not the test mass116, may be negatively detected (i.e., opposite in phase). Hence, insome examples, vibration signals from the sensors may match in magnitudebut may have opposite signs. The signals from the two sensors 110, 111may thus be combined by addition at block 118, rather than subtraction,to produce a greatly diminished streamer-motion response and asimultaneous increase in gain of the acoustic wave response.Alternatively, another cantilevered test mass in the first housing 114could be used. But, because the first sensor signal would also bereversed in polarity, it may need to be combined with the second sensorsignal by subtraction rather than addition.

As shown in FIG. 17, the sensor portion of the seismic system 19 can bemounted within a streamer cable 120 or within a cable-positioningdevice, such as a cable-leveling or cable-steering bird 122, rotatablyattached to the streamer by collars 124. As shown in FIG. 18, acable-positioning device 126 connected in line between fore and aftstreamer sections 128, 129 can house the sensor portion of the seismicsystem 19. Clearly, the sensors can be mounted in other devicesattachable in, on, or to a streamer, an ocean-bottom cable, or anautonomous node.

A tri-axis accelerometer with response to dc similar to the VectorSeissensor manufactured by ION Geophysical Corporation of Houston, Tex.,U.S.A., is suitable for many embodiments of the disclosure. Since thereis no dc component to the seismic wavelet, the dc response of the motionsensor is used to detect the orientation of the sensor relative togravity. One axis of the sensor is designed to be in the knownorientation of the streamer axis. Since the streamer axis orientation isknown and the gravity vector is measured, the orientation of the sensor,and thus the arriving sensed seismic wavelet, can be electronicallyrotated relative to gravity so that up-going seismic wavelets can beaccepted and down-going seismic wavelets rejected.

Any sensors that detect motion can be used. The sensors can be anymotion sensors responsive to position, velocity, or acceleration. Forinstance, a gimbaled first geophone, as described by Tenghamn et al. inU.S. Pat. No. 7,239,577, can be combined with a second geophone,packaged so that it has little or no response to an acoustic wave andthe same response to streamer motion, to achieve the desired result.Piezoelectric accelerometers can be used, as long as they have adequatesensor performance.

If the sensor cannot determine its own orientation, separate orientationsensors can be included in the sensor systems. Alternatively, mechanicalmeans—such as a gimbal system—can be used to fix the sensors in a knownorientation. Winged devices attached to the streamer, sometimes referredto as birds, can also be used to force the sensor into a desiredorientation.

In additional or alternative embodiments, the streamer or platform mayinclude multiple motion sensors that may assist or facilitate filteringnoise or readings of non-acoustic vibrations that may be present in thesignal received from the streamer. For instance, as illustrated in FIG.19A, a streamer 1900 may include a first elongated member, such as acable or stress member 1910 and a second elongated member, such as askin 1920. Collectively, the stress member 1910 and the skin 1920 maysecure multiple sensors, as described in further detail below.

While the particular shape and size of the streamer 1900 may vary fromone embodiment to the next, in some examples, the peripheral shape ofthe cross-section of the streamer 1900 may be defined by the skin 1920,which may at least partially enclose the stress member 1910.Furthermore, the streamer 1900 may have a generally elongated shape,such that a length of the streamer 1900 is substantially greater thanthe peripheral dimensions of the cross-sectional shape of the streamer1900.

As noted above, in some instances, the stress member 1910 may be a rope,a cable, or a similar. For example, the stress member 1910 may be a ropeof high strength fiber material. In one or more embodiments, the stressmember 1910 may include a metal cable (e.g., a braded or multi-strandcable), a solid cable, etc. Hence, in some instances, the stress member1910 may be substantially flexible. Alternatively, however, in at leastone embodiment, the stress member 1910 may be rigid and/or resilient. Inany case, in some examples, the stress member 1910 and/or the skin 1920may be placed in tension during operation of the streamer 1900 (e.g.,the streamer 1900 may be advanced in a medium and may be tensionedthereby).

It should be also appreciated that while in some instances reference ismade to a single stress member 1910, a streamer may include any suitablenumber of stress members (e.g., two, three, four, etc.), which may varyfrom one embodiment to the next. Similarly, the streamer 1900 may have asingle skin 1920 that, in some embodiments, may surround the stressmember 1910. Additionally or alternatively, however, a streamer mayinclude multiple skins.

In at least one embodiment, the streamer 1900 may include multiplemotion sensors. For example the streamer may include acoustic wavemotion sensors, such as motion sensor 1940 and/or additional sensors,such as motion sensors, 1930, 1950. Suitable motion sensor may vary fromone embodiment to another. Generally, the motion sensors 1930, 1940, or1950 may include any of the following types of sensors: accelerometers,geophones, capacitive sensors, optical sensors, etc.

Also, any or all of the motion sensors 1930, 1940, 1950 may be aunidirectional or multidirectional sensor. For example, the motionsensors 1930, 1940, 1950 may sense vibration along transverse y-axisand/or z-axis, each oriented transversely relative to a longitudinalx-axis that is aligned longitudinally with the stress member 1910 andskin 1920. Additionally or alternatively, the motion sensors 1930, 1940,1950 may sense vibration along the x-axis, y-axis, and/or z-axis of thestreamer. Moreover, any one of the motion sensors 1930, 1940, or 1950may be different from any other (e.g., different type, configuration,sensitivity ranges, etc.). For instance, the motion sensor 1930 and/ormotion sensor 1950 may be different from the motion sensor 1940.

In some embodiments, the motion sensor 1930 and/or the motion sensor1950 may be positioned and configured to sense vibration of the stressmember 1910 (e.g., the motion sensor 1930 and/or the motion sensor 1950may sense vibration of the stress member 1910 at or near locations oftheir respective positions). For instance, the motion sensor 1930 and/ormotion sensor 1950 may be positioned along a length of the stress member1910 and may sense vibration of the stress member 1910 at or near suchpositions. In an embodiment, the motion sensor 1930 and/or motion sensor1950 may be less sensitive to vibrations of the skin 1920 than tovibrations of the stress member 1910. In one example, the motion sensor1930 and/or motion sensor 1950 may be substantially insensitive tovibrations of the skin 1920, which may facilitate receiving readings orsignal output from the motion sensor 1930 and/or motion sensor 1950substantially representative of the vibration of the stress member 1910(i.e., the motion sensor 1930 and/or motion sensor 1950 may besubstantially unaffected by vibration of the skin 1920). Furthermore, inat least one embodiment, vibrations of the stress member 1910 may benon-acoustic vibrations (i.e., may substantially result from somethingother than acoustic waves or particle motion in the medium).

In additional or alternative embodiments, the motion sensor 1940 may bemore sensitive to the vibrations of the skin 1920 and the surroundingmedium than to the vibrations of the stress member 1910. For example,the motion sensor 1940 may be substantially isolated from and/orinsensitive to vibrations of the stress member 1910. In one or moreexamples, while the motion sensor 1940 may be substantially isolatedfrom the vibrations of the stress member 1910, such vibrations mayaffect or contaminate readings from the motion sensor 1940 (e.g., somevibrations from the stress member 1910 may be transferred to the motionsensor 1940). Generally, the motion sensor 1940 may be positioned at anysuitable location on the streamer 1900. In some instances, the motionsensor 1940 may be positioned between the motion sensor 1930 and themotion sensor 1950 (e.g., longitudinally along the same streamer, suchas along the streamer 1900). More specifically, examples may includemotion sensor 1940 positioned at a midpoint between the motion sensor1930 and the motion sensor 1950.

In any event, the streamer 1900 may include multiple motion sensors,such as motion sensors 1930, 1940, 1950, which may sense vibration ofthe stress member 1910 and/or skin 1920. Moreover, as described above,the motion sensor 1940 may more sensitive to acoustic waves, which maybe transmitted from medium, through the skin 1920, than to non-acousticwaves (e.g., which may be transmitted along the stress member 1910). Insome embodiments, the motion sensor 1940 may be sensitive substantiallyonly to the acoustic waves. It should be appreciated that thisdisclosure is not limited to a specific number of motion sensorsdescribed above, and the number of motion sensors included in a streamermay vary from one embodiment to another.

In one or more embodiments, the streamer 1900 may move relative toand/or within a medium. For example, the streamer 1900 may be advancedin a medium such as water (e.g., the streamer 1900 may be towed by amarine vessel along one or more sail lines). Hence, during such relativemotion of the streamer 1900 and the medium, the streamer 1900 may senseacoustic waves propagated through the medium. In some instances, theskin 1920 may be substantially transparent to the acoustic waves thatpropagate through the medium, such that the motion sensor 1940 may moveor vibrate in response to movement of medium's particles produced by theacoustic waves. In other words, in some embodiments, the motion sensor1940 may sense acoustic waves propagated in the medium.

Under some operational conditions, the motion sensor 1940 may sensevibration or movement of the stress member 1910 (e.g., stress member1910 may impart vibration or movement onto the motion sensor 1940). Suchvibration or movement sensed by the motion sensor 1940 may produce noisethat may contaminate readings related to the acoustic waves.Consequently, filtering such noise may produce filtered acoustic wavereadings that may represent acoustic waves more accurately thanunfiltered readings.

Generally, the motion sensors 1930, 1940, 1950 as well as any number ofadditional or alternative sensors, including pressure sensors,orientation sensors, etc., may be positioned at any number of suitablelocations on the streamer 1900. In one example, the motion sensor 1940may be positioned between the motion sensor 1930 and motion sensor 1950and may be separated by a distance therefrom along the streamer 1900.More specifically, in an embodiment, the motion sensor 1940 may bepositioned at a distance 1960 from the motion sensor 1930 and at adistance 1970 from the motion sensor 1950 (e.g., as measured fromimaginary centerlines or center points thereof). Also, in someinstances, the distance 1960 may be similar to or the same as thedistance 1970, such that the motion sensor 1940 is positionedapproximately midway between the motion sensor 1930 and motion sensor1950 (i.e., the sensors 1930 and 1950 are symmetrically positioned oneach side of sensor 1940). In other embodiments, the motion sensors1930, 1940, and 1950 may be positioned asymmetrically.

In some instances, positioning the motion sensor 1940 between the motionsensors 1930 and 1950 may facilitate calculating correction signals thatmay be used to modify unfiltered acoustic wave readings or signal outputfrom the streamer 1900 (e.g., from the motion sensor 1940), which mayinclude non-acoustic noise. More specifically, modifying unfilteredacoustic wave readings may produce filtered acoustic wave readings,which may more accurately represent actual acoustic waves. Hence, someembodiments may involve obtaining or receiving unfiltered acoustic wavereadings from the motion sensor 1940 and combining correction signalstherewith. In some examples, combining correction signals with theunfiltered readings may produce filtered acoustic wave readings that maybe substantially free of the non-acoustic noise. As described below, thecorrection signals may be generated by combing first and seconddetermined transfer functions with readings from the motion sensors 1930and 1950.

In some embodiments, the motion sensor 1930 and/or motion sensor 1950may move or vibrate together with the stress member 1910. Hence, thestress member 1910 may directly transfer vibration to the motion sensor1930 and/or motion sensor 1950 that, in turn, may produce and/or modifyan electrical signal that may provide information about the vibration,such as the amplitude and/or frequency of the vibration. For example, asillustrated in FIG. 19B, a streamer 1900 a may include a motion sensor1930 a mechanically coupled or otherwise mounted to a stress member 1910a. In some instances, the motion sensor 1930 a may be rigidly or fixedlyattached to the stress member 1910 a, such that the motion sensor 1930 amay have the same or similar amplitude and/or frequency of vibration asthe stress member 1910 a at the location of attachment of the motionsensor 1930 a to the stress member 1910 a. In at least some embodiments,the rigid connection between the stress member 1910 a and the motionsensor 1930 a may be imperfectly rigid, such that the stress member 1910a and the motion sensor 1930 a may have some amount of independentmovement relative to each other.

For the sake of simplicity, FIG. 19B illustrates the motion sensor 1930a and a relevant portion of the streamer 1900 a. It should beappreciated that, except as otherwise described herein, any of themotion sensors of the streamer 1900 a (e.g., motion sensor 1950 (FIG.19A)) may have the same or similar configuration and/or connection orcoupling with the streamer 1900 a as the motion sensor 1930 a. Moreover,except as otherwise described herein, the motion sensor 1930 a may besimilar to or the same as the motion sensor 1930 (FIG. 19A).

In one embodiment, a rigid or inflexible connection between the motionsensor 1930 a and the stress member 1910 a may reduce or eliminateasynchronous movement of the motion sensor 1930 a relative to the stressmember 1910 a. For instance, the motion sensor 1930 a may include ahousing 1931 a that may secure or house sensing components 1932 a (e.g.,one or more accelerometers). In some examples, the housing 1931 a (oranother portion of the motion sensor 1930 a) may include openings 1933 athat may accommodate the stress member 1910 a therein. In particular,the openings 1933 a may be tightly or snuggly fitted about the stressmember 1910 a.

In one or more embodiments, the stress member 1910 a may be press-fit orinterference fit in the openings 1933 a. Additionally or alternatively,the stress member 1910 a may be glued or otherwise adhered to the motionsensor 1930 a within the openings 1933 a. In some instances, the stressmember 1910 a may be fastened to the motion sensor 1930 a (e.g., withone or more fasteners, such as screws). Generally, the motion sensor1930 a may include or comprise any suitable material, which may varyfrom one embodiment to the next. For instance, the motion sensor 1930 amay include a plastic housing 1931 a. Hence, depending on the particularmaterials of the motion sensor 1930 a and/or stress member 1910 a, insome examples, the motion sensor 1930 a may be welded to the stressmember 1910 a (e.g., a plastic housing 1931 a may be ultrasonicallywelded to a stress member 1910 a).

Embodiments may include the motion sensor 1930 a fixedly or rigidlysecured to the stress member 1910 a or otherwise associated therewith tosense vibrations thereof.

Also, in at least one embodiment, the motion sensor 1930 a may be lesssensitive or less responsive to the vibration of the skin 1920 a, and insome embodiments, substantially insensitive thereto. For example, themotion sensor 1930 a may be spaced apart from the skin 1920 a, such thatthe skin 1920 a does not substantially contact the motion sensor 1930 a(e.g., as the skin 1920 a vibrates while affected by acoustic waves inthe medium). In other embodiments, and as shown in FIGS. 19A and 19B,the skin 1920 a is in contact with the motion sensor 1930 a, and may beglued or otherwise coupled to a housing for the motion sensor 1930 a.

As mentioned above, in some embodiments, the skin 1920 a may surroundthe stress member 1910 a. For instance, the skin 1920 a may have agenerally tubular or otherwise hollow shape. Hence, the motion sensor1930 a may be positioned inside the skin 1920 a.

As described in further detail below, the sensing components 1932 a ofthe motion sensor 1930 a may be operably coupled to one or moreprocessing devices (e.g., signal processor, controller, etc.). Inparticular, in some embodiments, the processing device may receivereadings or signal output from the sensing components 1932 a of themotion sensor 1930 a. Hence, in one example, the motion sensor mayinclude an electrical cable 1934 a, which may electrically couple thesensing components 1932 a to the processing device.

As mentioned above, the motion sensor 1940 (FIG. 19A) may be positionedand/or configured to be more sensitive to vibrations of the skin thanvibrations of the stress member. For instance, as shown in FIG. 19C, astreamer 1900 a may also include a motion sensor 1940 a that may be moresensitive to the vibration of the skin 1920 a than vibration of thestress member 1910 a. In some examples, as described in further detailbelow, the motion sensor 1940 a may be positioned along the streamer1900 a at a distance from the motion sensor 1930 a (FIG. 19B).

In some embodiments, the motion sensor 1940 a may be in contact withand/or mechanically coupled to the skin 1920 a, such that acousticvibration transmitted in the medium may be sensed by the motion sensor1940 a. For instance, the skin 1920 a may be stretched over a housing1941 a of the motion sensor 1940 a (e.g., the skin 1920 a may beelastically stretched over and/or glued to the motion sensor 1940 a in amanner that prevents or limits movement of the motion sensor 1940 arelative to the skin 1920 a). Furthermore, the motion sensor 1940 a maybe attached or otherwise secured to the skin 1920 a in a manner thatmovement of the skin 1920 a and/or particles of the medium inducecorresponding movement of the motion sensor 1940 a.

Similar to the motion sensor 1930 a (FIG. 19B), in one example, themotion sensor 1940 a shown in FIG. 19C may include one or more sensingcomponents 1942 a, which may sense motion or acceleration of the motionsensor 1940 a. For instance, the sensing components 1942 a may besecured inside the housing 1941 a of the motion sensor 1940 a. Also, insome embodiments, the housing 1941 a of the motion sensor 1940 a mayinclude openings 1943 a that may allow the stress member 1910 a to passtherethrough.

In at least one embodiment, the openings 1943 a may have sufficient orsuitable clearance relative to the stress member 1910 a. For example,the clearance between the openings 1943 a and the stress member 1910 amay prevent or limit contact between the stress member 1910 a and themotion sensor 1940 a during vibration of the stress member 1910 a.Consequently, the motion sensor 1940 a may be more sensitive orresponsive to the vibration of the skin 1920 a than vibration of thestress member 1910 a.

As described above, unfiltered readings of acoustic waves sensed by thestreamer may be processed to remove non-acoustic noise. For example,referring back to FIG. 19A, the streamer may include a motion sensor(e.g., motion sensor 1940) that senses acoustic waves through the skinof the streamer but may also be affected by vibrations from anothersource of non-acoustic vibration (i.e., noise). In some instances,non-acoustic vibrations readings may be vibrations of the stressmembers, which may contaminate the readings of the acoustic waves.Accordingly, correction signals may be used to modify the unfilteredreadings of the acoustic waves in a manner that may reduce or eliminatenoise from the unfiltered readings.

In some embodiments, correction signals may be calculated by obtainingtransfer functions of the vibrations propagating along the stressmember(s) between the motion sensors most sensitive to such vibration,such as, motion sensors 1930, 1950, and the motion sensor responsiblefor sensing the acoustic waves, such as motion sensor 1940. Inparticular, in an embodiment, the stress member may be vibrated in amanner that the vibrations propagate along the length thereof (with thevibrations being transverse to the length of the stress member in someembodiments) from the sensors sensitive to the vibration of the stressmembers, such as motion sensors 1930 and/or 1950 to the sensor beingmore sensitive to the acoustic waves than vibration of the stressmember, such as motion sensor 1940.

As described in further detail below, the sensing components 1942 a ofthe motion sensor 1940 a may be operably coupled to a processing device.In particular, in some embodiments, the processing device may receivereadings or signals output from the sensing components 1942 a of themotion sensor 1940 a. Hence, in one example, the motion sensor mayinclude an electrical cable 1944 a, which may electrically couple thesensing components 1942 a to the processing device.

FIG. 20 illustrates a schematic representation of a circuit 2000 that,in some embodiments, models the behavior of the stress member 1910 andskin 1920 as well as motion sensors 1930, 1940, 1950 (FIG. 19A). Forease of reference, electrical components and elements of the circuit2000 are also identified with reference numbers that correspond to theelements or component of the streamer 1900 (FIG. 19A), whose behaviorsuch electrical components may model. For example, behavior of elementsu_(v1) and u_(v2) in the circuit 2000 may correspond to readings orbehavior of the motion sensors 1930 and 1950, respectively, in thestreamer 1900.

In some embodiments, elements u_(v1) and u_(v2) may include respectivecapacitors m_(v1), m_(v2) connected thereto modeling masses of thesensors 1930, 1950. Similarly, the motion sensor 1940 may be representedby the element u_(a), which may include a capacitor m_(a) connectedthereto modeling a mass of the sensor 1940. In one embodiment, the skin1920 may be represented by impedance values Z₁, Z₂ and the stress member1910 may be represented by impedance values Z_(m) that representimpedance properties of the circuit 2000 at the illustrated locationsthereon. It should be also noted that, generally, elements of thestreamer 1900 may be represented according to generally acceptedconventions in dynamic modeling (e.g., masses may be represented ascapacitors, springs as inductors, dashpots as resistors, stress membersas impedances, velocity as voltage, force as current, etc.).

In some embodiments, the stress members 1910 may be vibrated (e.g.,artificially) at one or more suitable locations. In some instances,introduced vibration may facilitate determining or calculatingcorrection signals that may be used to correct unfiltered readings ofacoustic waves (i.e., readings of the motion sensor 1940). For example,the vibration may be first introduced at a first point on the stressmember 1910. More specifically, in an embodiment, the motion sensor 1930may be positioned between the first point and the motion sensor 1940. Inother words, the introduced vibration may propagate along the stressmember 1910 from the first point to the motion sensor 1930 and,subsequently, to the motion sensor 1940 and to the motion sensor 1950.

Similarly, the stress member 1910 may be vibrated at a second point,such that vibration propagates from the second point to the motionsensor 1950 and, subsequently, to motion sensor 1940 and to the motionsensor 1930. Accordingly, introducing vibration at the first and secondpoints may facilitate calculating or obtaining a first transfer functionfrom the motion sensor 1930 to the motion sensor 1940 and a secondtransfer function from the motion sensor 1950 to the motion sensor 1940.As described below in further detail, the first and second transferfunctions may be used to obtain correction signals that, in turn, may beused to filter noise from unfiltered readings of acoustic waves receivedor obtained from the motion sensor 1940.

In at least one embodiment, vibrations at the first and second points onthe stress member 1910 may be introduced artificially. For instance, oneor more vibration sources that, in some embodiments, may be operablyconnected (e.g., mechanically coupled) to the stress member 1910 and mayintroduce such artificial vibrations into the stress member 1910.Moreover, in some examples, a first vibration source may be connected ator near the first point of the stress member 1910 and a second vibrationsource may be connected at or near the second point of the stress member1910. As such, the first vibration source may be turned on to obtainfirst readings and may be turned off thereafter. Similarly, the secondvibration source may be turned on to obtain second readings and may beturned off thereafter. The first readings and the second readings areused to compute the first and second transfer functions. The circuit2000 models the first and second vibration sources as AC voltage sourcesVTG₁ and VTG₂, respectively.

Additionally or alternatively, vibrations could be introduced to thestreamer skin, to both the skin and the stress member, or generally toany one or more of the streamer components. It should be appreciatedthat locations of the vibration sources and motion sensors 1930, 1940,1950 are given as examples. The streamer may include any suitable numberof motion sensors and/or vibration sources, which may be positioned atany number of suitable locations along the streamer (e.g., along thestress member 1910, skin 1920, or other components of the streamer).

In addition to vibration sources, which may vibrate the stress member1910 (e.g., at the first and second points), the stress member 1910 mayalso experience environmental or natural vibration (i.e., vibrations notcaused by the vibration sources), which may originate at and/orpropagate from the same points as the vibration produced by thevibration sources, which may be at a predetermined frequency and/oramplitude. Natural vibrations may result from or may be caused by themovement of the streamer through the medium, vibrations from a vehicleadvancing the streamer (e.g., vibrations from a vessel or its motor),and so forth. Such natural vibrations may be added to the vibrationsartificially induced by the vibration sources. On the circuit 2000,natural vibrations are modeled as AC voltage sources N₁ and N₂.

The circuit 2000 may facilitate calculating or determining one or moretransfer functions. In some examples, the transfer function H_(v) may berepresented as:

$H_{v} = {\frac{1/{sm}_{a}}{{1/{sm}_{a}} + Z_{Th}} = \frac{1}{1 + {{sm}_{a}Z_{Th}}}}$where Z_(Th) = Z₁Z₂/(Z₁ + Z₂)m_(a) is the mass of motion sensor 1940, s is the Laplace transformvariable.

In one example, the circuit 2000 may be represented by a system ofequations in the frequency domain as follows:U _(1a) =U _(1ν1) G ₁ H _(ν) +U _(1ν2) G ₂ H _(ν)  Equation 1:U _(2a) =U _(2ν1) G ₁ H _(ν) +U _(2ν2) G ₂ H _(ν)  Equation 2:G ₂=1−G ₁  Equation 3:where a first transfer function from u_(v1) to u_(a) is represented byG₁H_(ν) and a second transfer function from u_(v2) to u_(a) isrepresented by G₂H_(ν), and G₁ and G₂ are defined as follows:

$G_{1} = \frac{Z_{2}}{Z_{1} + Z_{2}}$$G_{2} = \frac{Z_{1}}{Z_{1} + Z_{2}}$

The above system of equations may be solved for G₁, G₂, and H_(ν) in thefrequency and/or in the time domain in some embodiments. A samplesolution is provided below. In some instances, g₁ and g₂ are constantsclose to 0.5.

As described above, vibrating the stress member 1910 (and/or the skin1920 or other streamer components) by the first and/or second vibrationsources may facilitate obtaining transfer functions for filtering noisefrom the readings of the motion sensor 1940. For example, the readingsfrom motion sensor 1940 may be filtered using the following time domaincorrection equation:u=u _(a) −u _(ν1) *g ₁ h _(ν) −u _(ν2) *g ₂ h _(ν)where u is the filtered reading of acoustic vibrations and theoperation * is convolution in the time domain. In this equation, thecorrection signals u_(ν1)*g₁h_(ν) and u_(ν2)*g₂h_(ν) may be subtractedfrom the reading received or obtained from the motion sensor 1940, whichis represented in the circuit 2000 by u_(a).

As such, in some embodiments, correction signals may depend on or may bebased on the readings received from the motion sensor 1930 and/or motionsensor 1950. More specifically, readings received from the motion sensor1930 and/or motion sensor 1950 may be modified (e.g., using the transferfunctions described above) to obtain correction signals, which may besubtracted from the readings received from the motion sensor 1940.Moreover, such modifications or filtering of the readings received fromthe motion sensor 1940 may occur substantially in real time in some butnot all embodiments. For instance, as described below in further detail,a processing device may receive readings from the motion sensors 1930,1940, 1950 and may continuously filter the acoustic wave readings (e.g.,readings from the motion sensor 1940) in a manner described above afterthe transfer functions have been determined.

Any number of suitable vibration sources may be used to vibrate thestress member and/or other suitable portions of the streamer during acharacterization operation, in a manner that facilitates obtaining theone or more transfer functions. In some embodiments, the vibrationsource may be a motor with an eccentrically loaded shaft. An example ofa suitable motor assembly is illustrated in FIG. 21. As shown in FIG.21, a motor 2100 may include an eccentric mass 2110 that may be securedto or integrated with the shaft of the motor 2100. Accordingly, duringrotation, the shaft may be unevenly loaded, which may induce vibrationin the streamer in one or more dimensions, with such induced vibrationsbeing greater, and in some embodiments much greater than vibrationscaused by environmental forces such as the strumming of the streamercaused by towing the streamer behind a vessel.

In some examples, the vibration of the motor 2100 may be approximatelyperpendicular or transverse to a rotation axis of the shaft.Consequently, orienting the rotation axis of the shaft of the motor 2100approximately longitudinally or along a length of the streamer and/orthe stress member may vibrate the stress member transversely relative tothe length thereof (e.g., in the crossline and depth directions of thestreamer). More specifically, in some embodiments, the motor 2100 may becoupled or secured to the stress member in a manner that vibration ofthe motor 2100 is transferred to the stress member (e.g., at thelocation of attachment of the motor 2100 to the stress member). Also,securing the motor 2100 to the stress member in a manner that therotation axis of the shaft is perpendicular to the stress member mayproduce vibrations along the length of the stress member. Hence, thestreamer may include one or more vibration sources, such as the motor2100, that may vibrate the stress member along x-axis, y-axis, z-axis(FIG. 19A) or any combination thereof. In some embodiments, 1, 2, or 3vibration sources may be used to induce vibrations along 1, 2, or 3 axesor dimensions of the streamer.

The motor 2100 may be included in a motor assembly, which may couple(e.g., secure) the motor 2100 to one or more stress members, orotherwise associate the motor to the streamer in order to transmitvibration created by the motor to the stress member(s) and/or othercomponents of the streamer. For instance, the motor 2100 may be securedwithin a housing 2120 that may be secured to the stress members. In someembodiments, the housing 2120 may include one or more openings 2130 thatmay receive the corresponding stress members. More specifically, in atleast one embodiment, the openings 2130 may be secured to the stressmembers in the same or similar manner as described above in connectionwith the housing 1931 a (FIG. 19B).

In some instances, vibrations produced by the motor 2100 in the stressmember may correspond to the revolutions per minute (RPM) of the motor2100. Moreover, in one example, the RPM of the motor 2100 may be relatedto the voltage applied thereto (e.g., DC voltage). Consequently, varyingthe voltage applied to the motor 2100 may vary the vibrations producedby the motor 2100 in the stress member. In some embodiments, the motor2100 may be configured to vibrate the stress member at frequenciesbetween 1 Hz and 300 Hz. Consequently, during calibration of thestreamer, the motor 2100 may vibrate the stress member at any number ofsuitable frequencies such as, 10 Hz, 50 Hz, 100 Hz, 200 Hz, etc. In oneembodiment, the motor 2100 may be configured to generate vibrationsincluding a sweep of a plurality of vibration frequencies, which maysweep from 200 Hz down to 8 Hz in one specific example.

Additionally or alternatively, the motor 2100 may be positioned orlocated inside a holder 2140, which may include a first, second, andthird portions 2141, 2142, 2143. The holder 2140 together with the motor2100 may be secured inside an opening or a cavity in the housing 2120.The holder 2140 may seal the motor 2100 therein in a manner thatprevents or limits a medium from coming into contact with and/ordamaging the motor 2100 and/or components thereof. For example, O-ringsmay be positioned in the grooves 2144 and 2145 and may seal against thecavity in the housing 2120 that houses the holder 2140, therebypresenting or limiting the medium from entering the cavity in thehousing 2120 and/or the spaces between the first, second, and thirdportions 2141, 2142, 2143.

In some embodiments, the holder 2140 may be sealed inside the cavity ofthe housing 2120. For instance, the holder 2140 may facilitateelectrical connection to the motor 2100. Hence, sealing the holder 2140together with the motor 2100 in the cavity of the housing 2120 may alsoprotect or seal the electrical connections to the motor 2100 from themedium inside the streamer or in which the streamer may be at leastpartially submerged.

While FIG. 21 and corresponding text relate to a motor, which may be aDC motor, it should be appreciated that this disclosure is not solimited. For instance, the motor may be an AC motor, and the RPM of theAC motor may be controlled by controlling the frequency of the currentapplied thereto. Also, in some embodiments, the vibration source mayinclude a linear actuator that may move (e.g., cyclically) one or morepoints of the stress member to produce vibration along any one or moreof the x-axis, y-axis, z-axis, or any combination thereof. Suitablevibration sources may also include mechanical sources such as a tuningfork or a similar mechanism that may vibrate the stress member at apredetermined frequency. In any event, the vibration source may producevibration at one or more points on the stress member, which mayfacilitate determining one or more transfer functions for use infiltering non-acoustic vibrations sensed by the streamer.

Furthermore, in some instances the streamer may be calibrated (i.e.,characterized) prior to acquiring acoustic wave data, such as at thebeginning of a sail line before the seismic sources are activated. Forexample, the streamer may be advanced in the medium and transferfunctions may be obtained immediately before receiving acoustic wavereadings from the streamer, which may be modified or corrected by thecorrection using the one or more transfer functions, as described above.Alternatively, the streamer may be pre-calibrated (e.g., by amanufacturer) and transfer functions may be obtained in advance.Subsequently, the streamer may be at least partially submerged in amedium and/or moved therethrough to receive unfiltered readings ofacoustic waves, which may be modified or corrected by the correctionsignals obtained from the transfer functions that may be provided by themanufacturer.

Vibration sources, such as motor 2100, may additionally be used forother streamer quality control operations in some embodiments. Forexample, vibration sources may be used for one or more built inself-test (BIST) procedures of other streamer components that respond tostreamer vibration, or may be used for periodic calibration of otherstreamer components that respond to streamer vibration.

FIG. 22 illustrates a streamer 1900 b according to one embodiment.Except as otherwise described herein, the streamer 1900 b and itselements or components may be similar to or the same as any of thestreamers 1900, 1900 a (FIGS. 19A-19C) and their respective elements andcomponents. For example, the streamer 1900 b may include a stress member1910 b and a skin 1920 b that may be similar to or the same as stressmember 1910 and skin 1920 in FIG. 19A.

In some embodiments, the streamer 1900 b may also include motion sensors1930 b, 1940 b, 1950 b. The motion sensors 1930 b, 1940 b, and 1950 bmay have similar locations on the streamer 1900 b as the motion sensors1930, 1940, 1950 on the streamer 1900 (FIG. 19A). Moreover, in someinstances, the motion sensors 1930 b, 1940 b, 1950 b may be secured orotherwise coupled to the streamer 1900 b in a similar or the same manneras the motion sensors 1930, 1940, 1950 are coupled to the streamer 1900in FIG. 19A. The streamer 1900 b may include one or more vibrationsources, as described above. For example, the streamer 1900 b mayinclude a first vibration source 2200 and a second vibration source2210, each of which may be coupled to the stress member 1910 b and/orother streamer components. As such, in at least one embodiment, thefirst vibration source 2200 may vibrate the stress member 1910 b and ina manner that the vibration propagates from the first vibration source2200 toward the motion sensor 1930 b and further toward motion sensor1940 b and, in some embodiments, on towards motion sensor 1950 b.Likewise, second vibration source 2210 may vibrate the stress member1910 b and in a manner that the vibration propagates from the secondvibration source 2210 toward the motion sensor 1950 b and further towardmotion sensor 1940 b and, in some embodiments, on towards motion sensor1930 a. As noted above, vibrations produced by the first and/or secondvibration sources 2200, 2210 may be transverse relative to the length ofthe stress member 1910 b (e.g., in the crossline and depth dimensionsrelative to the streamer).

In some examples, the streamer 1900 b may also include one or morebuoyancy devices, such as buoyancy devices 2300. In at least oneembodiment, the skin 1920 b may be secured or coupled to the buoyancydevices 2300. Moreover, the buoyancy devices 2300 may be isolated orotherwise decoupled from the stress member 1910 b according to one ormore embodiments. In any event, the buoyancy devices 2300 may facilitatepositioning the streamer 1900 b at least partially within the externalmedium at a predetermined depth. For instance, the streamer 1900 b maybe secured to a vessel or other marine vehicle that may advance thestreamer 1900 b in a body of water. The buoyancy devices 2300 may floator position the streamer 1900 b at a desired depth relative to thesurface of the water during advancement of the streamer 1900 b.

In one or more embodiments, the streamer 1900 b may also include one ormore hydrophones 2400 that may be configured to measure a pressure wavein the medium. In one example, the response of the hydrophones 2400 maybe omnidirectional. Furthermore, the streamer 1900 b may include one ormore cable connectors 2500 that may be secured or connected to thestress member 1910 b and/or to the skin 1920 b. Additionally oralternatively, the cable connectors 2500 may secure the streamer 1900 bto a vehicle (e.g., a vessel) that may advance or tow the streamer 1900b in the medium, or other streamer section.

In some embodiments, the streamer 1900 b may be electrically coupled toone or more processing devices, such as a signal processor 2600 and/orto a controller 2700. More specifically, the signal processor 2600 mayreceive data or information from the motion sensors 1930 b, 1940 b, 1950b. The signal processor 2600 may also receive data or information fromthe hydrophones 2400. In some instances, the signal processor 2600 maycalculate suitable correction signals (e.g., using the determined one ormore transfer functions) and/or modify readings received from thestreamer 1900 b with one or more correction signals to produce filteredacoustic wave readings.

Additionally or alternatively, the signal processor 2600 and/or thecontroller 2700 may control and/or operate the vibration sources 2220and 2210 to vibrate the stress member 1910 b. For instance, the signalprocessor 2600 and or the controller 2700 may turn on and/or off thevibration sources 2200, 2210, may control the frequency and/or amplitudeof the vibration produced by the vibration sources to 2200, 2210, etc.Hence, in some embodiments, the signal processor 2600 and/or controller2700 may automatically calibrate the streamer 1900 b by, for example,calculating or obtaining one or more transfer functions for use ingenerating the correction signals. It should be appreciated that, whilethe signal processor signal processor 2600 and the controller 2700 aredescribed and illustrated as separate elements, in at least oneembodiment, the signal processor 2600 and the controller 2700 may beincluded in a single element, which may perform the functions or actsdescribed herein.

Moreover, the signal processor 2600 and/or the controller 2700 mayinclude a general purpose or a special purpose computer or computingdevice including computer hardware, such as one or more processors andsystem memory. Some embodiments include physical and othercomputer-readable media for carrying or storing computer-executableinstructions and/or data structures. In particular, one or more of theprocesses described herein may be implemented at least in part asinstructions embodied in a non-transitory computer-readable medium andexecutable by one or more computing devices (e.g., any of the mediacontent access devices described herein). In general, a processor (e.g.,a microprocessor) receives instructions, from a non-transitorycomputer-readable medium, (e.g., a memory, etc.), and executes thoseinstructions, thereby performing one or more processes, including one ormore of the processes described herein.

Computer-readable media can be any available media that can be accessedby a general purpose or special purpose computer system.Computer-readable media that store computer-executable instructions arenon-transitory computer-readable storage media (devices).Computer-readable media that carry computer-executable instructions aretransmission media. Thus, by way of example, and not limitation, someembodiments can comprise at least two distinctly different kinds ofcomputer-readable media: non-transitory computer-readable storage media(devices) and transmission media.

Non-transitory computer-readable storage media (devices) includes RAM,ROM, EEPROM, CD-ROM, solid state drives (“SSDs”), Flash memory,phase-change memory (“PCM”), other types of memory, other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium which can be used to store desired program code means inthe form of computer-executable instructions or data structures andwhich can be accessed by a general purpose or special purpose computer.

Further, upon reaching various computer system components, program codemeans in the form of computer-executable instructions or data structurescan be transferred automatically from transmission media tonon-transitory computer-readable storage media (devices) (or viceversa). For example, computer-executable instructions or data structuresreceived over a network or data link can be buffered in RAM within anetwork interface module, and then eventually transferred to computersystem RAM and/or to less volatile computer storage media (devices) at acomputer system. Thus, it should be understood that non-transitorycomputer-readable storage media (devices) can be included in computersystem components that also utilize transmission media.

Computer-executable instructions comprise, for example, instructions anddata which, when executed at a processor, cause a general purposecomputer, special purpose computer, or special purpose processing deviceto perform a certain function or group of functions. In someembodiments, computer-executable instructions are executed on a generalpurpose computer to turn the general purpose computer into a specialpurpose computer implementing elements of the disclosure. The computerexecutable instructions may be, for example, binaries, intermediateformat instructions such as assembly language, or even source code.Hence, in some embodiments, the signal processor 2600 and/or controller2700 may include computer-executable instructions that may cause thesignal processor 2600 and/or controller 2700 to perform one or more actsor functions described herein.

Those skilled in the art will appreciate that the invention may bepracticed in network computing environments with many types of computersystem configurations, including, personal computers, desktop computers,laptop computers, message processors, hand-held devices, multi-processorsystems, microprocessor-based or programmable consumer electronics,network PCs, minicomputers, mainframe computers, mobile telephones,PDAs, tablets, pagers, routers, switches, and the like. The inventionmay also be practiced in distributed system environments where local andremote computer systems, which are linked (either by hardwired datalinks, wireless data links, or by a combination of hardwired andwireless data links) through a network, both perform tasks. In adistributed system environment, program modules may be located in bothlocal and remote memory storage devices.

According to some embodiments, the signal processor 2600 and/orcontroller 2700, whether configured as a single component or multiplecomponents and whether configured as a special purpose computer or ageneral purpose computer, may receive vibration information from themotion sensors 1930 b, 1940 b, 1950 b during the calibration of thestreamer 1900 b. Moreover, the signal processor 2600 and/or thecontroller 2700 may receive or determine one or more transfer functionsand correction signals, which may be used to correct or modify thereadings of the motion sensor 1940 b to produce filtered readings of theacoustic waves, which may better represent the actual acoustic waves (ascompared with unfiltered readings) and/or which may be substantiallyfree of non-acoustic noise.

It should be appreciated that this disclosure is not limited to using asignal processor 2600 and/or controller 2700 to perform acts and/orfunctions described herein. For instance, any suitable processing devicemay be electrically coupled to one or more streamers to perform the actsand/or functions described herein. Accordingly, for example, one or moreprocessing devices may perform one or more acts illustrated in FIG. 23.

More specifically, in at least one embodiment, the processing device mayperform an act 3100 of receiving readings from first and second motionsensors corresponding to vibrations introduced by an artificialvibration source. For instance, the signal processor and/or thecontroller may receive signals from motion sensors that are sensitive tothe vibration of the stress member (e.g., motion sensor 1930, motionsensor 1940, and/or motion sensor 1950 (FIG. 19A)). The signal processorand/or the controller may also receive readings from additional motionsensors (e.g., other ones of the motion sensors 1930, 1940, and 1950).

The processing device may then perform an act 3110 of calculating one ormore correction signals based at least in part on the readings from thefirst and second sensors. In some embodiments, a correction signal maybe calculated using the transfer function derived from the readings ofthe first and second sensors in act 3100. For example, a correctionsignal may be calculated using subsequent readings from the motionsensor 1930 (i.e., readings obtained after those obtained in act 3100)and the transfer function as described above.

In act 3120, the processing device may receive readings from a thirdmotion sensor, such as motion sensor 1940 in FIG. 19A. Then, in act3130, the processing device may modify readings from the third motionsensor using one or more correction signals calculated in act 3110. Forexample, readings from the motion sensor 1940 (FIG. 19A) may be modifiedusing one or more correction signals determined based on the readingsfrom the motion sensor 1930 (FIG. 19A) (e.g., readings obtainedcontemporaneously with the readings from the motion sensor 1940 in act3120) and the one or more determined transfer functions. For example,the one or more correction signals may be subtracted from the readingsof the motion sensor 1940 (FIG. 19A). As suggested above, in someembodiments, the act 3100 may include receiving readings from two,three, or more motion sensors, such as 1930, 1940, and 1950, and one ormore transfer functions may be calculated based at least in part on thereadings received from these two, three, or more motion sensors, withthe one or more transfer functions in turn being used to calculate oneor more correction signals in act 3110.

In some embodiments, the signal processor and/or the controller maycause the artificial vibration sources vibrate the stress member in amanner that facilitates calibration of the streamer, which may includeobtaining one or more transfer functions. For instance, as shown in FIG.24, the signal processor and/or the controller may perform an act 3200of vibrating a portion of the streamer. In one example, the signalprocessor and/or the controller may activate one or more vibrationsources, which may vibrate a portion of the streamer, such as the stressmember.

The signal processor and/or the controller may also perform an act 3210of receiving readings from first and second (and possibly third) motionsensors. In one embodiment, the signal processor and/or the controllermay receive readings from the motion sensor 1930 and motion sensor 1940or from the motion sensor 1950 and motion sensor 1940, or from motionsensors 1930, 1940, 1950 (FIG. 24). Moreover, the processing device mayperform and act 3220 of calculating one or more transfer functions basedon the readings received from the first and second motion sensors (e.g.,as described above).

Accordingly, in some embodiments, as shown in FIG. 25, the processingdevice may perform any number of acts that may produce transferfunctions and correction signals for the streamer as well as obtain orreceive unfiltered readings from the streamer and filter such readings.More specifically, in at least one example, the processing device mayperform an act 3300 of producing first vibrations originating at a firstlocation of a portion of a streamer. For instance, the first vibrationsmay be produced or originated at the first location on the stressmember. In an embodiment, the first location may be near one or moremotions sensors (e.g., near the motion sensor 1930 (FIG. 19A)). Asdescribed above, the first location may be such that vibrations maypropagate along the stress member and toward two or more motion sensors(e.g., toward motion sensors 1930, 1950, and 1940 (FIG. 19A)).

The processing device may also perform an act 3310 of receiving readingsfrom motion sensors responsive to the first vibrations. For example,vibrations propagating along the stress member and toward two or moresensors may be sensed by such sensors and the readings from such sensorsmay be received by the signal processor and/or by the controller.

As described above, in some embodiments, two correction signals may beused to filter noise. Hence, in at least one example, the signalprocessor and/or the controller may perform an act 3320 of producingsecond vibrations originating at a second location of the portion of thestreamer and an act 3330 of receiving readings from motion sensorsresponsive to the second vibrations. In one or more embodiments, thesecond location may be different from the first location. For example,the second location may be such that the second vibrations propagatetoward motion sensor 1950 and subsequently toward motion sensor 1940 andmotion sensor 1930 (FIG. 19A).

The processing device may also perform an act 3340 of computing one ormore transfer functions. In an embodiment, the processing device maycompute the one or more transfer functions in a manner described above.

Moreover, the processing device may perform one or more additional actsthat may result in obtaining readings or values for acoustic waves. Insome instances, such acts may be performed after determining the one ormore transfer functions. For example, the processing device may performan act 3350 of receiving readings from at least one sensor in theabsence of the first and second vibrations. In other words, in someembodiments, the first and second vibrations may be absent (e.g., theprocessing device may turn off the first and second vibration sources),and the readings from the sensors may be received (e.g., from motionsensors 1930, 1940, 1950, or any combination thereof (FIG. 19A)). Suchreadings may be unfiltered and may represent acoustic vibrations sensedin the medium as well as non-acoustic vibrations or noise.

In some embodiments, the processing device may at least partially removenon-acoustic vibrations or noise or otherwise improve the unfilteredreadings. For instance, the processing device may filter readings byperforming an act 3360 of modifying the readings received in the absenceof the first and second vibrations by using the transfer functions toobtain correction signals (e.g., transfer functions obtained in the act3340). Hence, the processing device may reduce or eliminate noise orreadings of non-acoustic vibrations from the unfiltered readings, whichmay improve the quality or accuracy of the unfiltered readings, suchthat the filtered readings are more representative of the actualacoustic vibrations in the medium.

It should be appreciated that acts described herein may be performed inany suitable order. Furthermore, as described below in further detail,any two or more acts described herein may be performed iterativelyand/or repetitively or in a loop.

The streamer may be used in various commercial applications, includingbut not limited to oil and gas exploration. In one embodiment, a singlestreamer or multiple streamers may be attached to a marine vehicle toform an exploration vessel, such as an exploration vessel 2800illustrated in FIG. 26. Moreover, the exploration vessel 2800 mayinclude one or more signal processors and/or controllers that mayprocess signals or readings from the streamers, as described above.Additionally or alternatively, signal processor(s) and/or controller(s)may be physically located off the marine vehicle. In any event, in someembodiments, the exploration vessel 2800 may survey an area of a body ofwater by making one or more sail lines or passes as shown in FIG. 26.While a particular coverage pattern may vary from one embodiment to thenext, in one example, the exploration vessel 2800 may complete one ormore sail lines, such as by completing a first sail line 2810 andsubsequently completing a second sail line 2820, a third sail line 2830,a fourth sail line 2840, a fifth sail line 2850, and so on.

Hence, the exploration vessel 2800 may map the bottom of the body ofwater (e.g., ocean), which may include mapping or identifying structureslocated on the bottom of the body of water or subsea strata. Morespecifically, the exploration vessel 2800 may sense acoustic vibrationsthat may be reflected from the bottom of the body of water. For example,as described above, the acoustic energy may be sent or directed downwardthrough the water column and toward the ocean bottom, and may reflectoff underlying structures or subsea strata. As such, in someembodiments, the exploration vessel 2800 may produce acoustic wavereadings related to the structures underlying the body of water (i.e.,one or more streamers of the exploration vessel may provide reading orsignals to one or more signal processors and/or controllers), therebyidentifying and/or mapping such structures (e.g., relative to geographiccoordinates).

In some embodiments, the exploration vessel 2800 may recalibrate (i.e.,re-characterize) one or more of the streamers or obtain new or updatedtransfer functions whenever the exploration vessel 2800 changesdirection of movement, encounters water of different temperature ordensity, encounters a current, etc. For example, the exploration vessel2800 may recalibrate the streamer(s) or obtain new transfer functions atthe beginning of a sail line (e.g., at the beginning of each sail line).

Hence, as illustrated in FIG. 27, at least one embodiment may include anact 3400 of advancing the streamer in the first direction within themedium. During such advancement, the processing device may perform anact 3410 of obtaining first transfer function(s). In additional oralternative embodiments, during advancement of the streaming in thefirst direction, the processing device may also perform and act 3420 ofmodifying readings received from the streamer using the first transferfunction(s). In at least some instances, the act 3410 may be performedbefore the act 3420.

Moreover, an embodiment may include an act 3430 of advancing thestreamer in a second direction. Similarly, while the streamer isadvanced in the second direction, the processing device may perform anact 3440 of obtaining one or more second transfer function(s). Theprocessing device may also perform an act 3450 of modifying readingsreceived from the streamer using the second transfer function(s). Any ofthe acts described above may be performed in any sequence and/orrepeatedly in a loop. For example, as described above, the transferfunctions may be recalculated each time the streamer changes directionof advancement or movement.

Sample Solution

The following is a sample solution to the system of equations providedabove.

U_(1 a) = U_(1 v 1)G₁H_(v) + U_(1 v 2)(1 − G₁)H_(v)U_(2 a) = U_(2 v 1)G₁H_(v) + U_(2 v 2)(1 − G₁)H_(v) A  B  C$\frac{U_{1\; a}}{U_{2\; a}} = \frac{\left\lbrack {{U_{1\; v\; 1}G_{1}} + {U_{1\; v\; 2}\left( {1 - G_{1}} \right)}} \right\rbrack H_{v}}{\left\lbrack {{U_{2\; v\; 1}G_{1}} + {U_{2\; v\; 2}\left( {1 - G_{1}} \right)}} \right\rbrack H_{v}}$D  E  F$\frac{A}{D} = \frac{C + {G_{1}\left( {B - C} \right)}}{F + {G_{1}\left( {E - F} \right)}}$AF + G₁A(E − F) = DC + G₁D(B − C) G₁[A(E − F) − D(B − C)] = DC − AFG₁[AE − AF − DB + DC) = DC − AF$G_{1} = {\frac{{DC} - {AF}}{{AE} - {DB} + \left( {{DC} - {AF}} \right)} = \frac{1}{\frac{{AE} - {DB}}{{DC} - {AF}} + 1}}$$\begin{matrix}{G_{1} = \frac{1}{\frac{{U_{1\; a}U_{2\; v\; 1}} - {U_{2\; a}U_{1\; v\; 1}}}{{U_{2\; a}U_{1\; v\; 2}} - {U_{1\; a}U_{2\; v\; 2}}} + 1}} & {G_{2} = {1 - G_{1}}}\end{matrix}$$H_{1\; v} = \frac{U_{1\; a}}{{U_{1\; v\; 1}G_{1}} + {U_{1\; v\; 2}\left( {1 - G_{1}} \right)}}$$H_{2\; v} = \frac{U_{2\; a}}{{U_{2\; v\; 1}G_{1}} + {U_{2\; v\; 2}\left( {1 - G_{1}} \right)}}$$H_{v} = \frac{\left( {H_{1\; v} + H_{2\; v}} \right)}{2}$

In the foregoing, reference is made to embodiments of the invention.However, it should be understood that the invention is not limited tospecific described embodiments. Instead, any combination of the featuresand elements, whether related to different embodiments or not, iscontemplated to implement and practice the invention. Thus while theapparatuses and associated methods in accordance with the presentdisclosure have been described with reference to particular embodimentsthereof in order to illustrate the principles of operation, the abovedescription is by way of illustration and not by way of limitation.Various modifications and alterations to the described embodiments willbe apparent to those skilled in the art in view of the teachings herein.Those skilled in the art may, for example, be able to devise numeroussystems, arrangements and methods which, although not explicitly shownor described herein, embody the principles described and are thus withinthe spirit and scope of this disclosure. Accordingly, it is intendedthat all such alterations, variations, and modifications of thedisclosed embodiments are within the scope of this disclosure.

In methodologies directly or indirectly set forth herein, various stepsand operations are described in one possible order of operation, butthose skilled in the art will recognize that the steps and operationsmay be rearranged, replaced, or eliminated without necessarily departingfrom the spirit and scope of the disclosed embodiments. Further, allrelative and directional references used herein are given by way ofexample to aid the reader's understanding of the particular embodimentsdescribed herein. They should not be read to be requirements orlimitations, particularly as to the position, orientation, or use of thedisclosed embodiments.

Furthermore, in various embodiments, the disclosure provides numerousadvantages over the prior art. However, although embodiments may achieveadvantages over other possible solutions and/or over the prior art,whether or not a particular advantage is achieved by a given embodimentis not limiting of the disclosure. Thus, the described aspects,features, embodiments and advantages are merely illustrative and are notconsidered elements or limitations of the appended claims except whereexplicitly recited in a claim(s). Likewise, reference to “the invention”shall not be construed as a generalization of any inventive subjectmatter disclosed herein and shall not be considered to be an element orlimitation of the appended claims except where explicitly recited in aclaim(s).

What is claimed is:
 1. A method of sensing acoustic waves in a medium,the method comprising: receiving readings from a first motion sensor anda second motion sensor coupled to a streamer, wherein the streamercomprises an elongated cable member disposed in the medium and theacoustic waves reflect from structures underlying the medium; receivingreadings comprising unfiltered acoustic wave signals from a third motionsensor coupled to the streamer between the first motion sensor and thesecond motion sensor; and filtering noise from the readings receivedfrom the third motion sensor to generate a set of filtered data, whereinsaid filtering noise includes determining first and second transferfunctions corresponding to the streamer; and modifying the readingsreceived from the first motion sensor using the first transfer functionto generate a first set of modified readings; modifying the readingsreceived from the second motion sensor using the second transferfunction to generate a second set of modified readings; and modifyingthe readings received from the third motion sensor using the first andsecond sets of modified readings to generate the set of filtered data;wherein the unfiltered acoustic wave signals are processed to reducenon-acoustic noise and modified to produce the filtered acoustic wavesignal, which represents the acoustic waves propagating through themedium; and producing readings of the acoustic waves related to theunderlying structures, and identifying or mapping the structuresthereby.
 2. The method of claim 1, wherein said first and secondtransfer functions are determined prior to receiving the readings fromthe first, second, and third motion sensors.
 3. The method of claim 2,wherein said determining the first and second transfer functionscomprises: introducing first vibrations into the streamer using a firstvibration source; sensing the first vibrations using at least the firstand third motion sensors; introducing second vibrations into thestreamer using a second vibration source; sensing the second vibrationsusing at least the second and third motion sensors; and calculating thefirst and second transfer functions based on said sensing of the firstand second vibrations.
 4. The method of claim 1, wherein modifying thereadings received from the first motion sensor using the first transferfunction includes multiplying the readings received from the firstmotion sensor by the first transfer function, and modifying the readingsreceived from the second motion sensor using the second transferfunction includes multiplying the readings received from the secondmotion sensor by the second transfer function.
 5. The method of claim 4,wherein modifying the readings received from the third motion sensorcomprises subtracting said first and second sets of modified readingsfrom the readings from the third motion sensor.
 6. The method of claim1, further comprising: advancing the streamer through the medium in afirst direction, and determining the first and second transfer functionscorresponding to the streamer advancing in the first direction; andadvancing the streamer through the medium in a second direction, anddetermining third and fourth transfer functions corresponding to thestreamer advancing in the second direction.
 7. A method ofcharacterizing a marine seismic streamer according to claim 1, themethod further comprising: delivering first vibrations to a firstportion of the streamer, the first vibrations propagating toward thefirst motion sensor and further toward the second motion sensor;receiving first readings from the first motion sensor and from thesecond motion sensor corresponding to said first vibrations; deliveringsecond vibrations to a second portion of the streamer, the secondvibrations propagating toward the third motion sensor and further towardthe second motion sensor; receiving second readings from the secondmotion sensor and from the third motion sensor corresponding to saidsecond vibrations; and calculating one or more of the first and secondtransfer functions representative of the marine seismic streamer basedon the received first and second readings.
 8. The method of claim 7,wherein said one or more transfer functions are representative ofphysical characteristics of the streamer that determine how noise istransmitted along the streamer.
 9. A method of sensing acoustic waves ina medium according to claim 1, the method further comprising:transmitting first vibrations using a first vibration source positionedat a first location along the streamer; sensing the first vibrationsusing the first motion sensor and the second motion sensor; transmittingsecond vibrations using a second vibration source positioned at a secondlocation along the streamer; sensing the second vibrations using thethird motion sensor and the second motion sensor; characterizing thestreamer based at least in part on said sensing of the first and secondvibrations; and filtering acoustic wave measurements from the secondmotion sensor based at least in part on said characterizing of thestreamer.
 10. The method of claim 9, wherein said first and secondvibrations each include a sweep of a plurality of vibration frequencies.11. A method of sensing acoustic waves propagating in a medium, themethod comprising: receiving readings comprising unfiltered acousticsignals from a first motion sensor and a second motion sensor coupled toa streamer disposed in the medium; receiving readings comprisingunfiltered acoustic signals from a third motion sensor coupled to thestreamer between the first motion sensor and the second motion sensor,wherein the unfiltered acoustic wave signals include non-acoustic noise;filtering noise from the readings received from the third motion sensorto generate a set of filtered data, wherein said filtering noiseincludes determining first and second transfer functions correspondingto the streamer, modifying the readings received from the first motionsensor using the first transfer function to generate a first set ofmodified readings, modifying the readings received from the secondmotion sensor using the second transfer function to generate a secondset of modified readings, and modifying the readings received from thethird motion sensor using the first and second sets of modified readingsto generate the set of filtered data; and calculating a filteredacoustic wave signal based at least in part on the transfer functions,wherein the unfiltered acoustic wave signals are processed to reduce thenon-acoustic noise and modified to produce the filtered acoustic wavesignal, the filtered acoustic wave signal representing the acousticwaves propagating through the medium; producing readings of the acousticwaves related to the underlying structures and identifying or mappingthe structures thereby.
 12. The method of claim 11, further comprisingvibrating an elongated member of the streamer with a vibration sourceoperably coupled thereto and calculating the transfer functions at leastpartially based on information received from the first and second motionsensors spaced apart along the elongated member and sensitive tovibrations thereof, while the vibration source vibrates the elongatedmember.
 13. The method of claim 12, further comprising calculating thetransfer functions at least partially based on information received fromthe third motion sensor spaced apart from the first and second motionsensors along the elongated member and sensitive to the vibrationsthereof, while the vibration source vibrates the elongated member. 14.The method of claim 12, wherein the elongated member comprises a rope,cable, stress member or skin.
 15. The method of claim 11, furthercomprising vibrating an elongated member of the streamer with first andsecond vibration sources operably coupled thereto, wherein the firstvibration source is connected to the elongated member at a first pointand the second vibration source is connected to the elongated member ata second point.
 16. The method of claim 15, wherein the first positionmotion sensor is positioned between the first point and the secondmotion sensor and vibration introduced at the first point propagatesalong the elongated member from the first point to the first motionsensor and subsequently to the second motion sensor and to the thirdmotion sensor.
 17. The method of claim 16, wherein vibration introducedat the second point propagates from the second point to the third motionsensor and subsequently to the second motion sensor and to the firstmotion sensor.
 18. The method of claim 16, further comprisingintroducing the vibrations at the first and second points andcalculating a first transfer function from the first motion sensor tothe second motion sensor and the second transfer function from the thirdmotion sensor to the second motion sensor.
 19. The method of claim 16,further comprising advancing the streamer through the medium, whereinthe streamer is towed by a marine vessel along a sail line.
 20. Themethod of claim 19, further comprising advancing the streamer throughthe medium in a first direction and determining the first and secondtransfer functions corresponding to the streamer advancing in the firstdirection, and further advancing the streamer through the medium in asecond direction and determining third and fourth transfer functionscorresponding to the streamer advancing in the second direction.